Shot peening device

ABSTRACT

A shot peening device includes a turntable which rotates about a revolution axis, holding mechanism which moves with the turntable, pressure mechanism which compresses a coil spring, rotation mechanism which rotates the coil spring, projection mechanism which projects shots to the compressed coil spring, load cell which detects a load applied to the coil spring, and controller. A signal output from the load cell is input to the controller, and the controller detects a chronological change of the load applied to the coil spring during the shot peening.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation application of PCT Application No. PCT/JP2017/006931, filed Feb. 23, 2017 and based upon and claiming the benefit of priority from prior Japanese Patent Application No. 2016-032138, filed Feb. 23, 2016, the entire contents of all of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention described herein relate generally to a shot peening device which performs shot peening (stress shot peening) while a coil spring is being compressed.

2. Description of the Related Art

In order to increase durability of coil springs used in suspension springs of a suspension device of a vehicle, shot peening is well-known, which imparts a compressive residual stress to the coil spring. Patent Literature 1 (JP 2002-361558 A) discloses an example of a conventional shot peening device. The shot peening device thereof projects shots to the coil spring from a centrifugal accelerator (impeller) as the coil spring is conveyed. Patent Literature 2 (JP 2003-117830 A) discloses a conventional shot peening device. The shot peening device thereof compresses the coil spring and performs shot peening while the coil spring is stressed. That is, the shot peening device of the Patent Literature performs stress shot peening to impart a greater compressive residual stress to the coil spring. Furthermore, Patent Literature 3 (JP 2015-77638 A) discloses a shot peening device which performs shot peening on a rotating turntable in a state where the coil spring is compressed.

The shot peening device as in Patent Literature 1 simply hits shots to a coil spring, and thus, there is still a chance to increase the compressive residual stress of the coil spring. The shot peening device as in Patent Literatures 2 and 3 performs shot peening while a coil spring is compressed, and therein, the coil spring may be supported unstably depending on the shape of the coil spring (especially, on the shape of the end turn portion). Thus, stress shot peening may not be performed suitably because of the unstable coil spring.

BRIEF SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a shot peening device which can perform shot peening while a desired stress is applied to the coil spring.

According to an embodiment, a shot peening device includes: a turntable mechanism including a turntable which rotates around a revolution axis; a revolution mechanism which rotates the turntable mechanism; a holding mechanism which holds a lower end turn portion and an upper end turn portion of a coil spring while the coil spring is kept standing and moves around the revolution axis with the turntable; a rotation mechanism which rotates the holding mechanism around a rotation axis; a pressure mechanism which compresses the coil spring in a state where the coil spring is held by the holding mechanism; a load detector such as a load cell which detects a compressive load applied to the coil spring by the pressure mechanism; a projection mechanism which projects shots to the compressed coil spring; and a controller (for example, a personal computer) which detects a change in the load based on a signal output from the load detector.

The present invention can perform shot peening (stress shot peening) in a state where a desired stress is applied to a coil spring in order to impart the compressive residual stress to the coil spring, and thus, the coil spring of stable quality can be obtained.

For example, the load detector may be a load cell disposed in a load transfer path between the pressure mechanism and the holding mechanism. Furthermore, the controller may include means for storing a chronological change of the load. Or, the controller may include a display which displays a chronological change of the load.

The controller includes means for notifying that the load becomes out of an acceptable range while the projection mechanism projects shots to the coil spring. The controller may include a computer program which controls the pressure mechanism such that the load becomes a constant value while the projection mechanism projects shots to the coil spring.

Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention.

FIG. 1 is a perspective view of an example of a coil spring.

FIG. 2 is a flowchart of an example of the manufacturing process of the coil spring.

FIG. 3 is a schematic perspective view of a first shot peening device.

FIG. 4 is a perspective view of an example of a part of a conveyor and a transfer mechanism (robot).

FIG. 5 is a front view of a part of a second shot peening device of an embodiment.

FIG. 6 is a vertical cross-sectional view of the shot peening device of FIG. 5.

FIG. 7 is a horizontal cross-sectional view of the shot peening device of FIG. 5.

FIG. 8 is a perspective view of a lower side holder of the shot peening device of FIG. 5.

FIG. 9 is a front view of the lower side holder of the shot peening device of FIG. 5 and an end turn portion of the coil spring.

FIG. 10 is a flowchart showing an operation of the shot peening device of FIG. 5.

FIG. 11 is a graph showing an example of chronological change in the load during the warm stress shot peening.

FIG. 12 is a graph showing another example of chronological change in the load during the warm stress shot peening.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, a coil spring processing device including a shot peening device 50 of an embodiment will be explained with reference to FIGS. 1 to 12.

FIG. 1 shows an example of a coil spring 1. The coil spring 1 is formed of an element wire 2 wound helically. The coil spring 1 includes an end turn portion 1 a at its one end and an end turn portion 1 b at its other end. A relative relationship between end 1 c and other end 1 d of the coil spring 1 is constant corresponding to the type of the coil spring 1.

In the present application, a position of the coil spring 1 from the end 1 c around an axis C1 may be referred to as a position in a circumferential direction of the coil, or as a position in a winding direction. The coil spring 1 is, for example, a cylindrical coil spring; however, the coil spring 1 may be of various types such as a barrel-type coil spring, a hourglass-type coil spring, a tapered coil spring, an irregularly-pitched coil spring to conform to types of the suspension device. Furthermore, the end turn portions 1 a and 1 b may be formed in a negative pitch (negative pitch angle), or in a positive pitch (positive pitch angle).

FIG. 2 shows an example of a manufacturing process of the coil spring 1. In a forming process S1 of FIG. 2, the element wire 2 is formed helically by a coiling machine. In a heat treatment process S2, tempering and annealing of the wire 2 are performed to remove a distortion stress produced in the wire 2 by the forming process S1. For example, the wire 2 is heated to 400 to 450° C. and then is cooled slowly.

Furthermore, in a first shot peening process S3, first shot peening is performed in the warm using the remaining heat of the heat treatment process S2. In the first shot peening process S3, first shots are projected to the entire surface of the coil spring 1 in a process temperature of 250 to 300° C. by a first shot peening device 10 which is shown in FIG. 3. The first shot is, for example, a cut wire of which grain diameter is 1.1 mm. Note that a different shot peening device 10 may be used and a different shot size (for example, 0.87 to 1.2 mm) may be used. Through the first shot peening process S3, a compressive residual stress is produced to a relatively deep position from the surface of the coil spring 1. Furthermore, an oxide film (mill scale formed in the heat treatment) on the surface of the wire 2 is removed in the first shot peening process S3.

FIG. 3 shows a schematic example of the first shot peening device 10. The first shot peening device 10 includes a pair of rollers 11 and 12 and a shot projector (impeller) 13. Coil springs 1 are disposed on the rollers 11 and 12 in series in a position where the axis C1 is horizontal (laid horizontally). The coil spring 1 on the rollers 11 and 12 is rotated around the axis C1 to continuously move in the direction of arrow F1 in the figure. The shot projector 13 projects shots SH1 to the moving coil spring 1.

FIG. 4 shows a conveyor device 20 which is a part of the coil spring processing device and a robot 21 handling the coil spring 1. The conveyor device 20 continuously conveys the coil springs 1 in the direction of arrow F2. The robot 21 holds the coil spring 1 at both sides with an openable chuck 23 provided with the tip of an arm 22. The robot 21 is an example of a transfer mechanism used for moving the coil spring 1.

The robot 21 includes a function to store the positions of the ends 1 c and 1 d of the coil spring 1 held by the chuck 23 in a memory. The coil spring 1 held by the robot 21 is preliminarily maintained by means for positioning such as a jig such that the ends 1 c and 1 d are set to predetermined positions.

In a second shot peening process S4, second shot peening (warm stress shot peening) is performed by a shot peening device 50 of FIGS. 5 to 9. The second shot peening process S4 is performed in a temperature lower than that of the first shot peening process S3 (for example, warm zone of 200 to 250° C.) while the coil spring 1 is compressed. In the second shot peening process S4, second shots are projected to the entire surface of the coil spring 1. The size of second shot is less than that of the first shot used in the first shot peening process S3. The second shot is, for example, a cut wire of which grain diameter is 0.4 to 0.7 mm. Through the second shot peening process S4, the absolute value of the compressive residual stress in the proximity of the surface of wire 2 can be increased.

Then, a setting process S5 is performed if necessary. Furthermore, the coil spring 1 is coated in a coating process S6, and lastly, a quality inspection is performed in an inspection process S7 and the coil spring 1 is completed.

Now, the structure and operation of the second shot peening device 50 will be explained with reference to FIGS. 5 to 9. The second shot peening device 50 is a part of the coil spring processing device. The second shot peening device 50 performs shot peening in a warm zone between 200 and 250° C., for example while the coil spring 1 is kept standing. “The position where the coil spring 1 is kept standing” means that the axis C1 of the coil spring 1 is substantially vertical.

FIG. 5 is a front view showing a part of the second shot peening device 50. FIG. 6 is a vertical cross-sectional view of the second shot peening device 50. FIG. 7 is a horizontal cross-sectional view of the second shot peening device 50. The second shot peening device 50 includes a housing 51, turntable mechanism 52, projection mechanism 57 (shown in FIG. 6), first elevator mechanism 58, and second elevator mechanism 59. The projection mechanism 57 includes a first projection unit 55 and a second projection unit 56. The first elevator mechanism 58 and the second elevator mechanism 59 move the projection units 55 and 56 vertically.

The first elevator mechanism 58 and the second elevator mechanism 59 include, for example, servo motors 58 a and 59 a (shown in FIG. 6) of which rotation is controlled by a controller and ball screws 58 b and 59 b. The elevator mechanisms 58 and 59 move the projection units 55 and 56 independently and vertically at constant strokes Y1 and Y2 based on the direction and amount of rotation of the servo motors 58 a and 59 a.

As shown in FIGS. 6 and 7, a first chamber 61, a second chamber 62, and middle chambers 63 and 64 which are disposed between the chambers 61 and 62 are formed inside the housing 51. A coil spring inlet/outlet port 65 is formed in the first chamber 61. The coil spring inlet/outlet port 65 is an opening through which the coil spring 1 is put in and out the first chamber 61 from the outside the housing 51. The second chamber 62 is provide with a projection port 55 a of the first projection unit 55 and a projection port 56 a of the second projection unit 56. Shots SH2 are projected to the coil spring 1 from the projection ports 55 a and 56 a.

As shown in FIG. 7, partition walls 70 and 71 are provided between the first chamber 61 and the middle chambers 63 and 64. Partition walls 72 and 73 are provided between the second chamber 62 and the middle chambers 63 and 64. Seal walls 74 and 75 are formed in the middle chambers 63 and 64. The seal walls 74 and 75 keep the shots SH2 projected in the second chamber 62 from going to the first chamber 61.

As shown in FIG. 5, the turntable mechanism 52 includes a turntable 79, revolution mechanism 80 (shown in FIG. 5), first holding mechanism 81, and second holding mechanism 82. The turntable 79 rotates around a revolution axis X1 extending in the vertical direction. The revolution mechanism 80 is provided with a motor. The motor intermittently rotates the turntable 79, 180° at a time around the revolution axis X1 in either first direction R1 or second direction R2 (shown in FIG. 7). The holding mechanisms 81 and 82 rotate around the revolution axis X1 together with the turntable 79. The first holding mechanism 81 includes a lower side holder 81 a and an upper side holder 81 b. The lower side holder 81 a is disposed on the turntable 79. The upper side holder 81 b is disposed above the lower side holder 81 a to be opposed thereto. The second holding mechanism 82 also includes a lower side holder 82 a and an upper side holder 82 b. The lower side holder 82 a is disposed on the turntable 79. The upper holder 82 b is disposed above the lower side holder 82 a to be opposed thereto.

The first and second holding mechanism 81 and 82 are positioned 180° symmetrically about the revolution axis X1. In the rear side of the first and second holding mechanisms 81 and 82 on the turntable 79, a pair of backup plates 83 and 84 (shown in FIG. 7) are disposed.

A shifting prevention jig 85 is provided with each of the lower side holder 81 a of the first holding mechanism 81 and the lower side holder 82 a of the second holding mechanism 82. A lower end turn portion 1 a of the coil spring 1 can engage the shifting prevention jig 85. FIGS. 8 and 9 show the lower side holder 81 a of the first holding mechanism 81. The structure of the lower side holder 82 a of the second holding mechanism 82 is similar to that of the lower side holder 81 a of the first holding mechanism 81. Thus, the lower side holder 81 a of the first holding mechanism 81 will be explained with reference to FIGS. 8 and 9.

As shown in FIGS. 8 and 9, the shifting prevention jig 85 is provided with the lower side holder 81 a. The shifting prevention jig 85 includes a plurality of pawls (for example, three pawls) 85 a, 85 b, and 85 c. The pawls 85 a, 85 b, and 85 c are arranged to conform to the shape, pitch angle, and the like of the end turn portion 1 a such that the end turn portion 1 a of the coil spring 1 can be stably supported. For example, the pawls 85 a, 85 b, and 85 c are disposed on the lower side holder 81 a in its peripheral direction at regular intervals (for example, 90°). Note that the number of pawls of the lower shifting prevention jig 85 and the number of pawls of an upper shifting prevention jig 91 may be other than three. Furthermore, the pawls may be disposed at intervals at an angle other than 90°.

Guide grooves 86 a and 86 b are formed in a base member 86 of circular plate shape. The pawls 85 a, 85 b, and 85 c are movable along the guide grooves 86 a and 86 b. The pawls 85 a, 85 b, and 85 c are adjusted to a position corresponding to the end turn portion 1 a and the pawls 85 a, 85 b, and 85 c are fixed to the base member 86 by blots 87 (shown in FIG. 9). Height adjustment members 88 and 89 are provided between the base member 86 and the pawls 85 b and 85 c. The height adjustment members 88 and 89 have thicknesses T1 and T2 which correspond to the pitch angles of the end turn portions of the coil spring. Thus, even an end turn portion of negative pitch can be stably mounted on the pawls 85 a, 85 b, and 85 c. The pawls 85 a, 85 b, and 85 c each include a V-shaped groove 90 into which the end turn portion 1 a is inserted.

With the upper side holders 81 b and 82 b, a shifting prevention jig 91 corresponding to the upper end turn portion 1 b is provided. As in the lower shifting prevention jig 85, the upper shifting prevention jig 91 includes a plurality of pawls (for example, three pawls) conforming to the shape, pitch angle, and the like of the end turn portion 1 b. The upper end turn portion 1 b is held stably by the pawls. The upper shifting prevention jig 91 may be formed different from the lower shifting prevention jig 85 depending on the shape of the end turn portion 1 b.

The revolution mechanism 80 (shown in FIG. 5) rotates the turntable 79 around the revolution axis X1. That is, the revolution mechanism 80 intermittently rotates the turntable 79, 180° at a time around the revolution axis X1 in either first direction R1 or second direction R2 (shown in FIG. 7). When the first holding mechanism 81 is positioned in the first chamber 61, the second holding mechanism 82 is positioned in the second chamber 62. When the second holding mechanism 82 is positioned in the first chamber 61, the first holding mechanism 81 is positioned in the second chamber 62.

Furthermore, the shot peening device 50 includes, as shown in FIG. 5, a pressure mechanism 93 which compresses the coil spring 1. The pressure mechanism 93 includes presser units 94 and 95 which move the upper side holders 81 b and 82 b vertically. The presser units 94 and 95 include, for example, ball screws and servo motors. The presser units 94 and 95 can change a compression load (stress) applied to the coil spring 1 depending on the vertical movement amount of the upper side holders 81 b and 82 b. The presser units 94 and 95 may use fluid pressure as their drive source such as a hydraulic cylinder.

First and second presser units 94 and 95 include load cells 96 and 97, respectively. The load cells 96 and 97 are examples of load detectors. The load cells (load detectors) 96 and 97 detect a compression load applied to the coil spring 1 during the shot peening, and input an electrical signal related to the detected compression load to a controller 98. The first load cell 96 is disposed in a load transfer path between the first presser unit 94 and the upper side holder 81 b. The second load cell 97 is disposed in a load transfer path between the second presser unit 95 and the upper side holder 82 b.

The controller 98 includes a function (computer program) to detect a change of the load based on the outputs from the load cells 96 and 97. Furthermore, the controller 98 includes a function to notify that the load becomes out of an acceptable range during the shot peening. Furthermore, the controller 98 compares the load value output from the load cells 96 and 97 to a certain load value preset in the controller 98. Furthermore, the controller 98 includes a function to feedback the signals to the first and second presser units 94 and 95 such that a different between the output load value and the certain load value reaches zero, that is, a certain load can be applied to the coil spring 1.

The shot peening device 50 includes a rotation mechanism 100. The rotation mechanism 100 rotates the coil spring 1 around the rotation axes X2 and X3. The rotation axes X2 and X3 each extend in a vertical direction. The rotation mechanism 100 includes a lower rotator 101 and an upper rotator 102. The lower rotator 101 rotates the lower side holders 81 a and 82 a around the rotation axes X2 and X3. The upper rotator 102 rotates the upper side holders 81 b and 82 b around the rotation axes X2 and X3.

The lower rotator 101 and the upper rotator 102 each include a drive source of a timing belt and a servo motor. The controller 98 which controls the drive source rotates the lower rotator 101 and the upper rotator 102 in the same direction in synchronization at the same revolution rate. That is, the lower side holders 81 a and 82 a and the upper side holders 81 b and 82 b rotate in the same direction in synchronization at the same revolution rate. Furthermore, the lower side holders 81 a and 82 a and the upper side holders 81 b and 82 b can stop at a preset first rotation stop position or a preset second rotation stop position on the basis of the data preliminarily input in the controller 98. The first rotation stop position is, for example, a position suitable for the robot 21 to hand the coil spring 1 to the holding mechanisms 81 and 82. The second rotation stop position is, for example, a position suitable for taking the coil spring 1 from the holding mechanisms 81 and 82.

An information processor 110 such as a personal computer is connected to the controller 98. The information processor 110 includes an input device 111 used to input various data such as serial number of coil spring, display 112, and pointing device 113 such as a mouse. For example, various data (coil diameter, turn number, length, and wire diameter) related to the coil spring and data related to a load applied to coil spring during shot peening and the like can be input through the input device 111 or a storage medium 114.

The information processor 110 such as a personal computer functions as means for storing a chronological change of a load applied to the coil spring 1 during the shot peening. Furthermore, the display 112 of the information processor 110 functions as means for displaying the chronological change of the load during the shot peening. Note that the information processor 110 may include the functions of the controller 98.

FIG. 7 is a horizontal cross-sectional view of the first projection unit 55 and the second projection unit 56, viewed from the above. The first projection unit 55 includes an impeller (wing wheel) 121 and a distributor 122. The impeller 121 is rotated by a motor 120. The distributor 122 supplies shots SH2 to the impeller 121. The second projection unit 56 includes an impeller 126 rotated by a motor 125 and a distributor 127 which supplies shots SH2 to the impeller 126.

The first projection unit 55 is supported by a guide member 130 extending vertically to be movable in the vertical direction. The guide member 130 is provided with the side part of the housing 51. The first projection unit 55 reciprocates by the first elevator mechanism 58 (shown in FIG. 6) from a neutral position N1 to go over an ascend position A1 and a descend position B1. The second projection unit 56 is supported by a guide member 131 extending vertically to be movable in the vertical direction. The guide member 131 is provided with the side part of the housing 51. The second projection unit 56 reciprocates by the second elevator mechanism 59 from neutral position N2 to go over an ascend position A2 and a descend position B2.

FIG. 10 is a flowchart showing the operation of the shot peening device 50 of the present embodiment.

In step S10 of FIG. 10, the lower side holder 81 a of the first holding mechanism 81 is stopped in the first chamber 61. First coil spring 1 is set to (mounted on) the lower side holder 81 a by the robot 21 (shown in FIG. 4). The end turn portion 1 a mounted on the lower side holder 81 a is stopped by the shifting prevention jig 91 (shown in FIGS. 8 and 9). When the upper side holder 81 b descends, the coil spring 1 is compressed between the lower side holder 81 a and the upper side holder 81 b. At that time, the second holding mechanism 82 is positioned in the second chamber 62. The second holding mechanism 82 is in an empty state where no coil spring is mounted thereon. The coil spring 1 in the left of FIG. 5 is in a free state where no compression load is applied thereto. The length of the coil spring 1 in the free state (free length) is L1. The coil spring 1 in the right of FIG. 5 is in a state where it is compressed to length L2.

In step S11 of FIG. 10, the turntable 79 rotates 180° in a first direction. By the rotation, the coil spring 1 held by the first holding mechanism 81 is sent to the second chamber 62. At the same time, the second holding mechanism 82 is moved to the first chamber 61. In step S12, second coil spring 1 is set to the second holding mechanism 82.

In step S13, in the second chamber 62, the first coil spring 1 in the compressed state is rotated (turns on its axis) by the rotation mechanism 100 and shot peening is performed. That is, the first projection unit 55 and the second projection unit 56 moving vertically project shots SH2 to the first coil spring 1. The shot peening is performed while the stress is applied to the coil spring 1, and thus, a compressive residual stress which is effective to increase the durability of the coil spring 1 can be produced in a surface portion of the coil spring 1.

In step S14, the turntable 79 rotates 180° in a second direction. Thus, the coil spring 1 held by the first holding mechanism 81 is returned to the first chamber 61. Furthermore, the coil spring 1 held by the second holding mechanism 82 is sent to the second chamber 62.

In step S15, the upper side holder 81 b of the first holding mechanism 81 rises, and the first coil spring 1 held by the first holding mechanism 81 is taken by the robot 21. The first holding mechanism 81 becomes empty, and the robot 21 sets third coil spring 1 thereto. The upper side holder 81 b descends to compress the coil spring 1.

In step S16, in the second chamber 62, the second coil spring 1 in the compressed state is rotated (turns on its axis) by the rotation mechanism 100 and shot peening is performed. That is, the first projection unit 55 and the second projection unit 56 moving vertically project shots SH2 to the second coil spring 1.

In step S17, the turntable 79 rotates 180° again in the first direction. Thus, the coil spring 1 held by the first holding mechanism 81 is sent to the second chamber 62 and the second holding mechanism 82 is returned to the first chamber 61. The upper side holder 82 b of the second holding mechanism 82 rises, and then, the coil spring 1 held by the second holding mechanism 82 is taken by the robot 21. Next coil spring 1 is set by the robot 21 into the second holding mechanism 82 in the empty state. After that, the upper side holder 82 b descends to compress the coil spring 1. A series of steps S10 to S17 is repeated by the number of coil springs 1 (N times), and the shot peening of all coil springs 1 is completed.

FIG. 11 is a graph schematically showing an example of a chronological change of load (relationship between time and load) in warm stress shot peening performed by the shot peening device 50. For example, the shot peening device 50 performs the warm stress shot peening to the coil spring 1 held by the first holding mechanism 81, the coil spring 1 is compressed by the pressure mechanism 93. Between time t0 and t1 is an insensible zone of the load cell 96, and thus, the load is not detected. When the coil spring 1 is further compressed over the insensible zone, the load detected by the load cell 96 increases from Z1 to Z2. At time t2, the load reaches Z2 which is a target load, the pressure mechanism 93 is stopped. Then, the warm stress shot peening (projection of shots SH2) is started at time t3 while the compression (compression stroke) of the coil spring 1 is kept constant. The warm stress shot peening is performed until time t4 lapses.

When the compressive residual stress is produced in the coil spring 1 by the warm stress shot peening, the load tends to slightly increase by the compressive residual stress as shown in FIG. 11 with single-dotted line m1. However, a permanent set in fatigue occurs in the coil spring 1 which is compressed in the warm range, and thus, the load decreases as shown with dotted line m2 with some exaggeration. Thus, the load as a whole tends to slightly decrease as shown with solid line m3. The load as a whole (solid line m3) is determined a balance of the load increased by the occurrence of the compressive residual stress (line m1) and the load decreased by the permanent set in fatigue in the coil spring 1 (line m2). Thus, the load as a whole (line m3) may possibly be increased.

In the shot peening device 50 of the present embodiment, the load detected by the load cells 96 and 97 during the warm stress shot peening is constantly monitored by the controller 98. The information related to the detected load is displayed in the display 112. At the same time, the information is stored in a memory of the information processor 110 or in a storage medium 114. The information (a chronological change of load and the like) stored in the storage medium 114 or the like can be referred to anytime if necessary. If the detected change of the load is within an acceptable range, it is determined that the warm stress shot peening is performed properly while a certain load is applied to the coil spring.

As explained above, the data related to chronological change of load are stored in the internal memory of the information processor 110 or the storage medium 114 during the warm stress shot peening is performed by the shot peening device 50. This process proves that the warm stress shot peening to the coil spring is performed properly, that is, the quality of coil spring.

A permanent set in fatigue of the coil spring during the shot peening may be a problem. In that case, as shown in FIG. 11 with two-dotted line m4, the signals output from the load cells 96 and 97 are sent back to the pressure mechanism 93 such that the load during the warm stress shot peening can be constant. Then, the presser units 94 and 95 are driven in the real time to change the stroke of compression, and thus, the warm stress shot peening can be substantially performed in the constant compression load.

FIG. 12 is a graph schematically showing another example of a change of load occurring during the warm stress shot peening. As shown in FIG. 12 with solid line m5, the load may suddenly drops at time t5 to be out of the acceptable range (threshold value). In that case, the coil spring 1 may possibly be dismounted from the first holding mechanism 81 or the second holding mechanism 82 during the shot peening. Thus, if such a steep drop of load is recognized, it is determined that the warm stress shot peening is not performed properly, and the coil spring is handled as a defective.

Furthermore, as shown in FIG. 12 with dotted line m6, if the load changes periodically during the shot peening, the coil spring 1 may possibly be held unstably. If such a change in load is recognized, it is determined that the warm stress shot peening is not performed properly, and the coil spring is handled as a defective.

When the present invention is achieved, models, structures, and arrangement of the elements of the shot peening device can be arbitrarily changed. For example, the controller processing the signals output from the load cells may be a personal computer or may be an information processor storing computer program developed specifically for the shot peening device. Furthermore, a load detector other than the load cell may be used.

Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents. 

What is claimed is:
 1. A shot peening device comprising: a turntable mechanism including a turntable which rotates around a revolution axis; a revolution mechanism which rotates the turntable mechanism; a holding mechanism which holds a lower end turn portion and an upper end turn portion of a coil spring while the coil spring is kept standing and moves around the revolution axis with the turntable; a rotation mechanism which rotates the holding mechanism around a rotation axis; a pressure mechanism which compresses the coil spring in a state where the coil spring is held by the holding mechanism; a load detector which detects a load applied to the coil spring by the pressure mechanism; a projection mechanism which projects a shot to the compressed coil spring; and a controller which detects a change in the load based on a signal output from the load detector.
 2. The shot peening device of claim 1, wherein the load detector is a load cell disposed in a load transfer path between the pressure mechanism and the holding mechanism.
 3. The shot peening device of claim 1, wherein the controller includes means for storing a chronological change of the load.
 4. The shot peening device of claim 2, wherein the controller includes means for storing a chronological change of the load.
 5. The shot peening device of claim 1, wherein the controller includes a display which displays a chronological change of the load.
 6. The shot peening device of claim 2, wherein the controller includes a display which displays a chronological change of the load.
 7. The shot peening device of claim 3, wherein the controller includes a display which displays a chronological change of the load.
 8. The shot peening device of claim 4, wherein the controller includes a display which displays a chronological change of the load.
 9. The shot peening device of claim 1, wherein the controller includes means for notifying that the load becomes out of an acceptable range while the projection mechanism projects shots to the coil spring.
 10. The shot peening device of claim 2, wherein the controller includes means for notifying that the load becomes out of an acceptable range while the projection mechanism projects shots to the coil spring.
 11. The shot peening device of claim 1, wherein the controller controls the pressure mechanism such that the load becomes a constant value while the projection mechanism projects shots to the coil spring.
 12. The shot peening device of claim 2, wherein the controller controls the pressure mechanism such that the load becomes a constant value while the projection mechanism projects shots to the coil spring. 